Eighteen cases of atresia coli were observed in the University of Illinois Holstein herd from 1974 through August 1983 in 2367 births. The overall incidence of the abnormality was .76% during this period. All of the affected calves were related to one another, and 15 of them were inbred. More calves were born with atresia coli from dams in which pregnancy was diagnosed prior to d 41 of gestation than from dams in which pregnancy was diagnosed later. Early pregnancy diagnosis in dams may contribute, but was not essential, to atresia coli in Holstein calves. Thirteen of the affected calves were males. The proportion of calves aborted or born dead among the affected calves was greater than expected from the incidence of aborted or born dead among contemporary calves.